The present invention relates to a multi-stage process for the continuous preparation of thermoplastically processable polyurethanes.
Thermoplastic polyurethanes (TPU) have been known for a long time. They are of industrial importance because of the combination of high-quality mechanical properties with the known advantages of inexpensive thermoplastic processability. A wide range of variation of the mechanical properties can be achieved by using various chemical builder components. An overview of TPUs, their properties and uses is given, for example, in Kunststoffe 68 (1978), pages 819 to 825 or in Kautschuk, Gummi, Kunststoffe 35 (1982), pages 568 to 584.
TPUs are built up from linear polyols, which are usually polyester- or polyether-polyols, organic diisocyanates and short-chain diols (i.e. chain lengtheners). A diversity of combinations of properties can be established in a targeted manner via the polyols. Catalysts can additionally be added to accelerate the formation reaction. The builder components can be varied in relatively wide molar ratios to establish the desired properties. Molar ratios of polyols to chain lengtheners of 1:1 to 1:12 have proved suitable. These result in products in the range from 70 Shore A to 75 Shore D.
Thermoplastically processable polyurethane elastomers can be built up either stepwise (e.g. by prepolymer metering process), or by simultaneous reaction of all the components in one stage (e.g. by one-shot metering process).
TPUs can be prepared continuously or discontinuously. The best known preparation processes are the belt process (see, for example, GB-A 1 057 018) and the extruder process (see, for example, DE-A 19 64 834, DE-A 23 02 564 and DE-A 20 59 570). In the extruder process, the starting substances are metered into a screw reactor, undergo polyaddition there and are converted into a uniform granule form. The extruder process is comparatively simple, but has the disadvantage that the homogeneity of the products prepared in this manner is inadequate for many uses because the mixing and reaction progress simultaneously. In addition, because of the limited reaction space and the limited metering possibilities, the variability in the targeted use of various polyols is limited.
The two-stage process described in, for example, EP-A 0 571 828, in which the prepolymer is built up from a polyol and a diisocyanate in a targeted manner in a tube reactor before the extruder, provided an improvement in respect of a targeted and controlled preparation of TPUs with improved processing properties. The TPU formation is concluded in the subsequent extruder with the addition of the chain lengthener. On the basis of the optimum conditions in each process stage, TPUs can thus be prepared in a targeted and controlled manner.
However, for many end-uses, it is not sufficient to use only one polyol in preparing the TPU. Particular combinations of TPU properties can be achieved by the simultaneous use of different polyols. An example which may be mentioned is the combination of polyester-polyols and polyether-polyols and the resulting advantages. By addition of particular phosphorus-containing polyols which can be built in, the flame resistance of the resultant TPU can be improved without adversely influencing other properties.
If the sometimes chemically very different polyols are reacted simultaneously by, e.g. the prepolymer process, or even together with the chain lengthener by, e.g. the one-shot process, with the diisocyanate in a continuous preparation process, tacky, poorly processable TPUs are probably obtained due to the reaction conditions no longer being optimum for all the starting substances.
The object of the present invention was thus to provide an economically favorable continuous process with which it is possible to prepare readily processable, homogeneous, non-tacky TPUs in an industrially simple manner.
Surprisingly, it has been possible to achieve this object by a continuous multi-stage preparation process.